Yttria stabilized zirconia (YSZ) electrolytes, subjected to high current densities, develop significant damage. This paper characterizes the damage that develops from the anode side, in YSZ used in an oxygen pump configuration. The damage is in the form of vacancy loops, precipitates formed on dislocations, pore development, and grain boundary fracture. The damage initiates at elevated current densities, well below the decomposition potential of YSZ. A mechanism, involving mobile oxygen interstitials, is proposed.